The present invention relates to a method of programming nonvolatile memory devices.
In recent years, there is an increasing demand for nonvolatile memory devices which can be electrically programmed and erased and does not need to be periodically refreshed.
A nonvolatile memory device is a device that enables electrical program/erase operations, and performs the program and erase operations by changing the threshold voltage of a cell as electrons are moved by a strong electric field applied to a thin oxide layer.
The nonvolatile memory device typically includes a memory cell array in which cells for storing data are arranged in matrix form, and a page buffer for writing data into specific cells of the memory cell array or reading data stored in a specific cell. The page buffer includes bit line pairs coupled to specific memory cells and a register for temporarily storing data to be written into the memory cell array or reading data from a specific cell of the memory cell array and temporarily storing the read data. It also includes a sense node for sensing the voltage level of a specific bit line or a specific register, and a bit line select unit for controlling whether the specific bit line and the sense node have been coupled or not.
In a program operation of this nonvolatile memory device, there occurs a phenomenon in which a program speed changes according to the characteristics of cells and in the number of program/erase cycles. Meanwhile, the program operation of a nonvolatile memory device is carried out using an incremental step pulse program (ISPP) method. It is necessary to set a program start voltage in consideration of the program speed of each cell.